"Frog and Toad Are Friends," a favorite in our household, contains an early reference to "snail mail" (sort of). Frog gives a letter to a snail, who takes four days to deliver it a short distance. The book was published in 1970, so the term "snail mail" didn't exist — at least, not the way we know it today.
The story depicts the pleasure of receiving a letter from a friend (or rather, anticipating a letter — since they spend most of the time waiting for the snail to arrive).
When I was a kid, I would fill out cards in magazines asking for more information from the advertisers — just so I could get letters. I also had a cherished book called "SWAK" that showed how to get free stuff through the mail (the process often entailed sending someone a SASE). Needless to say, the book appears to be out of print.
Now that we're approaching end of days for the U.S. Postal Service, I imagine today's kids will never know what SWAK and SASE even stand for.